That's because, Trump said, he wanted the Secretary of State to join him in Washington.
Democrats warned against interference in the probe.
"Sessions did not do the thing he could have done to better protect Rosenstein, and through Rosenstein, the Mueller investigation", Vladeck said.
More alarmingly for Democrats, Mr Whitaker himself had outlined a specific way to stymie the Russian Federation probe. Whitaker criticized both aspects previous year, and as he is now taking over oversight of the probe, he could try to strangle it, or even rescind Mueller's appointment.
How could Whitaker impede Mueller without firing him?
"I'm profoundly concerned about it", he said. Mitt Romney, who just won election to the U.S. Senate from Utah, also tweeted that the Mueller probe should be allowed to proceed "to its conclusion unimpeded".
The special counsel regulation under which Mr Mueller was appointed gives the attorney general or acting attorney general authority to fire Mueller only for "good cause", such as misconduct, dereliction of duty, incapacity or conflict of interest.
Mueller declined to comment, according to his spokesman, Peter Carr.
"I am committed to leading a fair Department with the highest ethical standards, that upholds the rule of law, and seeks justice for all Americans", Whitaker said. He added that it had "been a privilege" to work under Sessions.
"Trump's actions are being compared with those of U.S. President Richard Nixon in one of the most controversial episodes of the Watergate scandal, the Saturday Night Massacre", said organizers. "Whitaker was Sessions' chief of staff and had been considered for a variety of jobs in the Trump administration, including the No. 2 post at Justice or as White House counsel".
Could a Mueller firing be challenged on legal grounds? "In fact, Mr Whitaker told CNN a year ago that a future attorney general could limit Mueller by reducing the budget so low that his investigations grinds to nearly a halt".
Matthew previously ran as a republican for U.S. Senate and state treasurer.
Donald Trump plunged Washington into turmoil over suspicions that he's trying to kill the Russian Federation collusion probe.
He went on to call on Mr Rosenstein to "order Mueller to limit the scope of the investigation" or risk the inquiry starting "to look like a political fishing expedition".
President Trump's forcing out of Jeff Sessions has cast doubt on the future of Robert Mueller's investigation-especially since Sessions' replacement, Acting Attorney General Matthew Whitaker, has always been a critic of the probe. "There is no mistaking what this means, and what is at stake: this is a constitutionally perilous moment for our country and for the president".